of the presence of forces acting and dragging in different directions within the solid mass. LECTURE III. COHESION--CHEMICAL AFFINITY. We will first return for a few minutes to one of the experiments made yesterday. You remember what we put together on that occasion--powdered alum and warm water; here is one of the basins then used. Nothing has been done to it since; but you will find on examining it, that it no longer contains any powder, but a multitude of beautiful crystals. Here also are the pieces of coke which I put into the other basin--they have a fine mass of crystals about them. That other basin I will leave as it is. I will not pour the water from it, because it will shew you that the particles of alum have done something more than merely crystallise together. They have pushed the dirty matter from them, laying it around the outside or outer edge of the lower crystals--squeezed out as it were by the strong attraction which the particles of alum have for each other. And now for another experiment. We have already gained a knowledge of the manner in which the particles of bodies--of solid bodies--attract each other, and we have learnt that it makes calcareous spar, alum, and so forth, crystallise in these regular forms. Now, let me gradually lead your minds to a knowledge of the means we possess of making this attraction alter a little in its force; either of increasing, or diminishing, or apparently of destroying it altogether. I will take this piece of iron [a rod of iron about two feet long, and a quarter of an inch in diameter], it has at present a great deal of strength, due to its attraction of cohesion; but if Mr. Anderson will make part of this red-hot in the fire, we shall then find that it will become soft, just as sealing-wax will when heated, and we shall also find that the more it is heated the softer it becomes. Ah! but what does _soft_ mean? Why, that the attraction between the particles is so weakened that it is no longer sufficient to resist the power we bring to bear upon it. [Mr. Anderson handed to the Lecturer the iron rod, with one end red-hot, which he shewed could be easily twisted about with a pair of pliers.] You see, I now find no difficulty in bending this end about as I like; whereas I cannot bend the cold part at all. And you know how the smith takes a piece of iron and heats it, in order to render it soft for his purpose: he acts upon our principle of lessening the adhesion of the particles, although he is not exactly acquainted with the terms by which we express it.